1. Field of the Invention
The present invention relates to a powder of unagglomerated metallic particles. More particularly, the present invention relates to a powder of unagglomerated metallic particles having an average diameter of about 1-100 nm and the process for making the same. Additionally, the powder of unagglomerated metallic particles can be formed into a lyophilized form which upon reconstitution maintains the average diameter of between about 1-100 nm wherein the particles remain unagglomerated.
2. Description of the Related Art
Nanoscale (particle diameter of 1-100 nm) metallic particles are important components in the development of catalytic, sensor, aerosol, filter, biomedical, magnetic, dielectric, optical, electronic, structural, ceramic and metallurgical applications. Nanoscale metallic particles exhibit volume and surface effects which are absent in the same material with dimensions in the micron range (i.e. 0.1 micron&lt;particle diameter &lt;1 micron).
Metallic nano-particles using polymerized phospholipid membranes of phosphatidylcholines and mixtures of phosphatidylcholines and phosphatidylhydroxyalkanols have been reported. See Markowitz et al., Polymerized Phospholipid Membrane Mediated Synthesis of Metal Nanoparticles, Langmuir, pp. 4095-4102, (November 1994), incorporated herein by reference in its entirety and for all purposes. The use of polymerized vesicles as templates for the synthesis of unagglomerated particles presents several difficulties. For example, the use of polymerized vesicles (as templates for the synthesis of unagglomerated nanoparticles) involves the diffusion of reactants across the polymerized membrane. Thus, such polymerized vesicles limit the process of forming unagglomerated nanoparticles to only those reactants which can cross the polymerized vesicle membrane.
In addition, such unpolymerized vesicles are relatively unstable to the asymmetric addition of high concentrations of bivalent ions (e.g. metal ions). Thus, a process wherein unpolymerized vesicles are used for the formation of size controlled unagglomerated nanoparticles is desired. The advantages of using nonpolymerized vesicles (as templates for the formation of metallic nanoparticles of between about 1-100 nm) are several including:
(i) self-assembly of vesicles in the appropriate solvent; PA1 (ii) a simplified process for the production of metallic nanoparticles; PA1 (iii) a more cost effective process for the production of metallic nanoparticles; and PA1 (iv) metallic nanoparticles readily redisperse upon hydration after lyophilization. PA1 (1) forming a first dispersion of surfactant vesicles in the presence of catalytic first metal ions at a first concentration, said surfactant being lipid A or a mixture of lipid A and lipid B at a ratio of lipid A: lipid B, said lipid A having the structure (A): ##STR1## said lipid B having the structure (B): ##STR2## wherein R, R', R" and R'" are selected from the group consisting of saturated aliphatic, unsaturated aliphatic, haloaliphatic, aromatic, haloaromatic compounds and mixtures thereof, wherein R'" further comprises a positively charged group, said first concentration being not more than about a stoichiometrically sufficient concentration to bind to said lipid (A), said first dispersion having a pH; PA1 (2) forming a second dispersion by controlling said pH of said first dispersion to between about 5-7; PA1 (3) forming a third dispersion by mixing a metallization bath containing second metal ions with said second dispersion; and PA1 (4) forming a fourth dispersion by incubating said third dispersion at a temperature sufficient to reduce said second metal ions to metal particles having an average diameter between about 1-100 nm. Additionally, another step of PA1 (5) forming a fifth dispersion by separating said metal particles and said surfactant from said fourth dispersion may be carried out.